1. Field of the Invention
The present invention relates to improved anode catalyst materials for use in fuel cells. More specifically, the present invention relates to CO tolerant anode catalyst materials for use in the low temperature proton exchange membrane fuel cell (PEMFC) or the direct methanol fuel cell (DMFC).
2. Background Art
Fuel cells based on polymer electrolytes present a promising approach to a stable, low-temperature energy source for e.g. vehicles. A substantial amount of interest has been focused on the H2 fuel cells as well as the direct methanol fuel cells.
One of the promising developments in H2 fuel cells is the proton exchange membrane fuel cell (PEMFC) which is based on a polymer proton conducting membrane acting as the electrolyte, and which operates at low temperatures, typically 80xc2x0 C.
The principle of such a H2 fuel cell can be outlined as follows: The fuel cell comprises an anode and a cathode, which are physically separated by a membrane of a polymer electrolyte. Hydrogen is supplied to the anode and atmospheric oxygen is accessible to the cathode. If electrically conducting cords are connected to the anode and cathode, respectively, and a circuit thereby is established (e.g. by connecting an external power consuming unit) the fuel cell will begin operating.
At the anode, the supplied hydrogen is dissociated to protons and electrons according to the reaction:
H2xe2x86x922H++2exe2x88x92
The electrically insulating polymeric electrolyte membrane prevents the flow of electrons from the anode through the membrane to the cathode, whereas the produced protons readily flow through the membrane from the anode to the cathode.
At the cathode, supplied oxygen will be reduced according to the following reaction:
O2+4H++4exe2x88x92xe2x86x922H2O
Thus, the net result in operating the fuel cell will be the conversion of hydrogen (supplied at the anode) and oxygen (supplied at the cathode) to water and electrical energy.
The H2 supplied to this kind of fuel cell is usually produced from natural gas or from methanol or other liquid fuels by a reformer system. Hydrogen obtained this way inevitably contains small amounts of CO impurities.
The purpose and the requirement of the anode catalyst materialxe2x80x94to dissociate hydrogen to protons and electronsxe2x80x94obviously make the type of anode material a crucial feature. At present, the most successful materials have proven to be platinum or platinum-based alloys. These materials work perfectly for pure hydrogen. However, these prior art materials for use as anode catalysts in low-temperature proton exchange membrane fuel cells suffer from a severe drawback. Carbon monoxide shows a high affinty towards these materials compared to the affinity of hydrogen. As a result, when using the hydrogen originating from reformer systems, the specific, active sites on the surface of the prior art platinum or platinum-based alloy catalyst materials will be occupied by carbon monoxide molecules and will accordingly over time reduce the accessibility of hydrogen molecules to these sites. Thus, as a result, the fuel cell will have a smaller efficiency. This situation is also termed xe2x80x9cpoisoningxe2x80x9d of the catalyst.
The direct methanol fuel cell (DMFC) is currently also subject to considerable interest and research. In this system methanol is supplied to the anode and atmospheric oxygen is available to the cathode. The processes taking place in a DMFC are rather complicated; however, carbon monoxide is formed at the anode in an intermediate reaction step. Thus, also in this kind of fuel cells the problems associated with the poisoning of the anode are present.
The present invention is applicable to the H2 fuel cell systems as well as the direct methanol fuel cell systems.
Numerous examples of materials based on platinum for use as catalysts are known in the art:
WO 00/55928 (Gorer; Symyx Technologies, Inc.; published on Sep. 21, 2000) relates inter alia to an improved noble ternary metal alloy composition for a fuel cell catalyst, which alloy contains Group VIII metals especially Pt, Ru and Ni. The PtRuNi alloys have a bulk composition in the range of about 40 to about 70 atomic % of Pt, about 30 to about 50 atomic % of Ru and less than about 30 atomic % of Ni. The fuel used consists of hydrogen or preferably of methanol. The alloys may be used in PEMFC. The CO tolerance is rather satisfactory, but the alloys suffer from the drawback that the Ni content will be corroded as a result of the acidic ambience of the anode.
WO 00/54346 (Gorer; Symyx Technologies, Inc.; published on Sep. 14, 2000) relates inter alia to an improved ternary nobel metal alloy catalyst composition for use in electrochemical reactor devices, which alloy consists essentially of from about 20 to about 60 atomic % Pt, from about 20 to about 60 atomic % Ru, and from about 5 to about 45 atomic % Pd, the atomic ratio of Pt to Ru being between about 0.6 and about 1.8. The useful fuel is preferably selected from saturated hydrocarbons, garbage off-gas, oxygenated hydrocarbons, fossil fuels, mixtures thereof and is most preferably methanol. The alloys may be used in a PEMFC. The CO tolerance is considerably less satisfactory than for the former prior art catalyst material.
U.S. Pat. No. 5,922,487 (Watanabe et al.; Tanaka Kikinzoku Kogyo, Masahiro Watanabe and Stonehart Associates, Inc.; published Jul. 13, 1999) discloses an anode electrocatalyst for a fuel cell comprising an alloy essentially consisting of at least 1 to 60 atomic % Sn or 33 to 55 atomic % Mo or 30 to 60 atomic % Ge and the balance of one or more noble metals selected from Pt, Pd and Ru. Sn, Ge and Mo has the ability of depressing the poisoning of the noble metal with CO. The use of these electrocatalysts in solid polymer electrolyte fuel cells is contemplated. The CO tolerance is just as satisfactory as for the first-mentioned prior art catalytic material, but also here corrosion problems prevail under the acidic ambience of the anode.
U.S. Pat. No. 5,208,207 (Stonehart et al.; Tanaka Kikinzoku Kogyo K. K., Stonehart Associates Inc.; published on May 4, 1993) relates to a catalyst, which comprises an inorganic support and a ternary alloy essentially consisting of 10-50 atomic-% Pt, 10-50 atomic % Pd and 10-50 atomic-% Ru. In the specification it is stated that the support is restricted to inorganic porous substances. Especially preferred support materials are silica, alumina or carbon. Although it is stated that the catalyst possesses excellent anti-CO-poisoning properties, these properties are considerably less satisfactory than the properties of the first-mentioned prior art catalytic material.
U.S. Pat. No. 5,013,618 (Francis J. Luczak; International Fuel Cells Corporation; published May 7, 1991) refers to a noble metal ternary alloy catalyst for use in fuel cell electrodes and other catalytic structures. The catalyst exhibits increased mass activity and stability. The catalyst comprises a ternary alloy of Pt, Ir, and a metal selected from the group consisting of Fe, Cr, Co, Ni, V, Ti and Mn. However, only phosphoric acid fuel cells are mentioned in this publication. The use of this catalyst takes place at a temperature being much higher than the low-temperature fuel cell operating temperature.
An object of the present invention is to provide anode catalyst materials with improved CO tolerance and/or corrosion resistance for use in fuel cells. More specifically, this object is to provide ternary and quaternary anode catalyst materials for use in low-temperature proton exchange membrane fuel cells (PEMFC) or in direct methanol fuel cells (DMFC) or for other fuel cell systems, where CO poisoning is a problem. Another object of the present invention is to provide anodes for fuel cells comprising said inventive catalyst materials. A third object of the present invention is to provide fuel cells comprising such anodes. Yet another object of the present invention is a method for the manufacture of the inventive catalyst materials and, finally, a further object of the present invention is a method for the generation of electrical power by using a fuel cell according to the invention.
It is a well-known feature of ruthenium, that attempts to alloy this metal with other metal(s) lead to the segregation of the other metal(s) to the surface. Accordingly, this makes ruthenium-based alloys a perfect substrate material (in the following we refer to the Ru-based alloy as the Ru substrate) for catalyst materials as only the surface of such a material is active in the catalytic process. As mentioned above, Pt is an ideal metal for use as catalysts for PEMFC when ultra pure hydrogen is used as a fuel, but the platinum atoms get occupied by carbon monoxide even if only trace amounts of CO are present in the hydrogen feed.
Thus, the basic idea of the present invention is to modify the surface properties of platinum and thereby adjusting the absorption energies of hydrogen and carbon monoxide, respectively, in such a way that the rate of proton formation is increased and the occupation of carbon monoxide is depressed; leading to catalyst materials that are far better than the catalysts of prior art. Thus, according to the invention this modification of the surface properties is accomplished by utilising contributions from the underlying substrate metal as well as contributions from modifying surface metal atoms other than platinum. As an alternative, substrate metal Os is taken into consideration.
It has surprisingly been found that a fuel cell anode catalyst material having a surface being comprised of:
a composition Mx/Pty/Sub;
wherein
M is selected from the group of elements Fe, Co, Rh and Ir; or
M represents M1m+M2n;
M1 and M2 being different from each other, are selected from the group of elements consisting of Fe, Co, Rh, Ir, Ni, Pd, Cu, Ag, Au and Sn;
Sub represents the substrate metal being selected from Ru and Os, which substrate metal may be present at the surface of the anode material in an amount of less than 25% and will be alloyed in the bulk with the metal M and Pt;
x is a number in the range of 0.7-1.3;
y is a number in the range of 0.7-2.3; and
m and n are each a number in the range of 0.7-2.3, lead to catalysts showing improved CO tolerance when used as catalyst material for anodes in the low temperature proton exchange membrane fuel cell (PEMFC), especially when using reformate hydrogen as fuel or when used as catalyst materials for anodes in the direct methanol fuel cell (DMFC).
In the present application x, y, m and n signify the relative occurrence of the respective elements in the surface, not including the substrate metal. Preferably, x is a number in the range of 0.8-1.2, and especially x is 1. Preferably, y is a number in the range of 0.8-2.2, and especially y is 2. Preferably, m and n are each a number in the range of 0.8-2.2, and especially m and n are 2.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.